I'd go and take a look at your local Evans or Cycle Surgery and get proper advice from there on everything you need. Both staff tend to be very knowledgeable and cyclists themselves:
http://www.evanscycles.com
http://www.cyclesurgery.com
I find pretty much across the board with any sport that requires an outlay the best thing is to be honest with yourself. If you're a natural athletic person and you think you'll train hard when you get the bug, there's absolutely no point getting entry level as you'll be annoyed with it in no time.
I reckon about £600 - £700 is a decent amount to spend on a bike if you think you're going to really go for it, but you can get away with spending £400 - £500 and still get a really nice bike. If you decided you wanted a Hybrid or Mountain Bike you could probably spend a little less again.
£700 as a magic figure should do it though.
In terms of gear, SPD/Clip in pedals will help you to ride longer whilst working more of your muscles as you can use your quads to pull through the back end of the pedal. Good investment if you're going to stick with it.
Padded shorts might be wise
and a decent cycling top with pockets on the back so you can stuff them full of phones, tools, and a few sugary things
in case you hit the wall in the middle of buttfudge nowhere.
Cage and a water bottle. A small pump that you can carry, an inner tube, a repair kit and a little saddlepack. Always wear a helmet on the road, drivers can be brick to cyclists in this country and lights if you are going to possibly get caught out for light.
In truth though, you can probably get away with just a helmet and a water bottle... Depends how tekkers you want to be and if walking with a bike for 5 miles bothers you if you get a flat in the middle of nowhere.